This invention relates to thermal ink-jet (TIJ) printers, and more particularly to improvements in the pens used therein.
TIJ printers typically include a TIJ pen which includes a reservoir of ink coupled to the TIJ printhead. One type of pen includes a polymer foam disposed within the print reservoir so that the capillary action of the foam will prevent ink from leaking or drooling from the printhead. In such a pen, a fine mesh filter is typically provided in the fluid path between the reservoir and the printhead to trap particles before reaching the printhead and thereby interfering with printhead operations. This foam pen includes a vented air delivery system, wherein as ink is drawn from the ink reservoir during printing operations, air enters the reservoir via a separate vent opening.
The TIJ pen 50 illustrated in FIG. 1 and described in the referenced co-pending applications affords many benefits for the printing system built to utilize it. The pen is thin which directly reduces the required width of the printer carriage and subsequently the total width of the printer. The ink delivery system is simple and efficient. Ink is contained within a reservoir formed by two pieces of thin polyethylene bag material that have been thermally bonded to a compatible plastic material on the frame 60. Two pistons and a spring inside the bag provide back-pressure to prevent ink from drooling out of the printhead, i.e., the ink is maintained under negative pressure within the reservoir. The frame 60 is made of two different plastic materials. One material is an engineering plastic forming the external surfaces and providing structural support and the second material provides the fluid path for the ink and is suitable for thermal attachment of the bag material. The thin metal sidecovers 70 and 80 protect the inside components, add considerable rigidity to the system, and allow for a high degree of volumetric efficiency (volume of deliverable ink compared to the external volume of the pen). Sidecovers made from a metal having a surface such a pre-painted or PVC clad material are used to cover the springbag and other components of this TIJ pen.
Negative pressure on the ink within the reservoir will tend to draw air bubbles through the printhead and the fluid path into the reservoir when exposing the pen to shock. A problem with negative pressure pens such as that shown in FIG. 1 is the leakage of air bubbles through the printhead and into the ink reservoir, thus reducing and ultimately equalizing the pressure on the ink in the reservoir. As the negative pressure is reduced or eliminated, ink will readily drool from the printhead when the pen is subjected to even minor shocks during handling or operation.
It is therefore an object of this invention to provide a solution to the problem of leakage of air bubbles into an ink reservoir under negative pressure.
A further object is to provide a thermal ink-jet pen having a negative pressure ink reservoir with an air check valve disposed in the ink fluid path between the ink reservoir and the printhead.